This invention relates to a method of preparing refractory silicon carbide composite materials having a minimum of free silicon by the infiltration of carbonaceous preforms with alloyed silicon melts.
The formation of silicon carbide through the infiltration of carbonaceous preforms with liquid silicon is known. This process was first developed in the 1950's by the United Kingdom Atomic Energy Authority (UKAEA) as a means of bonding coarse silicon carbide grains together into a refractory body, hence the name "self-bonded silicon carbide" or "reaction-bonded silicon carbide" (RBSC). The reaction of liquid silicon with solid carbon forms SiC in a rapid and exothermic process.
The General Electric Company has also developed a family of related silicon/silicon carbide materials, known under the tradename SILCOMP*, which are fabricated by infiltrating fibrous carbon materials with liquid silicon, to obtain silicon carbide substantially retaining the fibrous habit of the preform. The fabrication and properties of these materials are described in papers by R.L. Mehan, Journal of Materials Science, 13, 358, 1978, and W.B. Hillin et al., Ceramic Bulletin, 54[12], 1054, 1975. In U.S. Pat. Nos. 3,325,346 and 3,348,967, Hucke described fabrication of hard carbide materials by infiltrating carbon frameworks with liquid metal melts that react with the framework. Thus, carbide materials other than silicon carbide can also be fabricated by melt infiltration and reaction.
For the synthesis of silicon carbide materials, the speed of the process can be one advantage, and results from the very good wetting of carbon by molten silicon, the low viscosity and rapid infiltration of the Si melt, and the self-heating nature of the reaction. As will be discussed below, however, sometimes the reaction rate is too rapid, resulting in dissolution of the carbon skeleton or, in some cases stopping the infiltration process. Another advantage of the RBSC process is that the temperatures required are much lower than usual methods of consolidating SiC for applications at high temperature. Processing is usually done near the melting point of silicon (1410.degree. C.), whereas hot-gressing and sintering of SiC requires at least 1800.degree. C. A third advantage is that reaction-bonding can directly yield a fully-dense material. Finally, it is possible to prepare objects that have a minimum of shape and dimensional changes relative to the starting preform--it is a near-net shape, near-net dimension process.
The work of Hucke improved on the microstructures and strengths attainable in RBSC by controlling the characteristics of the carbon preform. See, "Process Development for Silicon Carbide Based Structural Ceramics," Army Materials and Mechanics Research Center Report No.. DAAG46-80-C-0056-P004, Jan. 1983. By pyrolysis of an orqanic precursor, a microporous and uniform carbon preform can be obtained. Upon liquid Si infiltration of these preforms, a finer-drained microstructure of SiC with correspondingly higher strength (up to 714 MPa) was achieved than in the case of coarser drained particulate carbon starting materials.
However, there has always been one serious deficiency to the reaction-bonded silicon carbide process which has prevented use of these materials at high temperatures (&gt;1410.degree. C.). i.e., above the melting point of silicon. In order to obtain full infiltration to useful dimensions, it has been necessary to use preforms with an excess of porosity, such that free silicon remains after full reaction. The high density of RBSC is therefore achieved by filling excess void space with free silicon. It is not sufficient to leave an excess of carbon in the preform, for often the reaction does not do to completion, leaving unreacted carbon particulates which can degrade the mechanical properties due to differential thermal expansion between carbon and SiC. These difficulties are well-documented in the Hucke work cited above, in the general literature, and in a recent U.S. Pat. No. 4,477,493. These limitations hold true even when fine, microporous carbons are used. The amount of free silicon is usually 5-15%, and causes the high temperature strength and creep resistance of RBSC to degrade rapidly above 1410.degree. C. Thus, while the processing temperatures of RBSC are low relative to alternative processes such as hot-pressing and sintering, the ultimate use temperatures are not any higher than processing temperatures.